The operation of a 90 degree V-six, four stroke cycle internal combustion engine having offset crankpins on each throw is such that movement of the piston crank mechanism causes an inertial unbalance comprising an eccentric primary rotating unbalance couple turning in the direction of the crankshaft rotation and an eccentric secondary rotating unbalance couple turning opposite to the direction of crankshaft rotation.
A substantial portion of the primary unbalance can be offset by the application of counterweights to the crankshaft in accordance with common practice. However, in many applications, the residual primary and the secondary unbalance is not offset and generally requires the use of a complex or bulky apparatus to cancel the effects of these latter inertial forces.
The invention essentially eliminates the above objections and disadvantages by generating first and second engine order of vibration couples that counter the couples produced by motion of the engine's pistons. This is accomplished by the use of a pair of coaxially mounted balance shafts mounted in the valley of the V formed between the longitudinal banks of cylinders of a V-six engine, for example, for compactness and simplicity. Both shafts are rotated by the camshaft in the same direction at different speeds, and use eccentric counterweights to generate moments that oppose the engine's inertial unbalance couples.